Liquid Crystals

Liquid Crystals
Syllabus Statements
• C.6.1 Describe the meaning of the term liquid crystals.
• C.6.2 Distinguish between thermotropic and lyotropic liquid
crystals
• C.6.3 Describe the liquid-crystal state in terms of the
arrangement of the molecules and explain thermotropic
behaviour.
• C.6.4 Outline the principles of the liquid crystal display
device
• C.6.5 Discuss the properties needed for a substance to be
used in liquid-crystal displays
C.6.1 Describe the meaning of the term
liquid crystals.
• Liquid crystals are fluids that have physical
properties (e.g. electrical, optical, elastic) that
are dependent on the molecules orientation
relative to some fixed axis.
• In crystals, the molecules are fully oriented
and fixed in position.
• In an isotropic liquid the molecules are
arranged completely randomly. The properties
of the liquid are the same in all directions.
• Liquid crystals lie between these extremes.
• They are a mesophase.
Smectic or nematic?
• Smectic liquid crystals still have the molecules
arranged in layers.
• Individual molecules can move within layers,
but are always roughly parallel.
A smectic liquid crystal
• A nematic liquid crystal doesn’t have layers.
• On average the molecules point in the same
direction
A nematic liquid crystal
• You’ve probably spotted that liquid crystals
tend to have rod shaped molecules.
• These are usually elongated organic
molecules.
• Some common examples are DNA, spider silk,
cellulose.
• You need to be able to draw some examples
Another example:
• MBBA
• N-(4-Methoxybenzylidene)-4-butylaniline
C.6.2 Distinguish between thermotropic and lyotropic
liquid crystals
• Thermotropic liquid crystals are pure
substances that exhibit L.C. behaviour over a
temperature range between solids and liquids.
• Note that by implication, substances that
show L.C. behaviour are not always in the
correct phase to show L.C. behaviour.
• Some specific examples are:
• Cholesterol Myristate
• Solid below 71°C ; Clear liquid above 86°C
• In between, it is a cloudy liquid – a liquid
crystal
• Biphenyl nitriles (a family of molecules!)
•
•
•
•
“5CB”
Real name 4-cyano-4-n-pentylbiphenyl
Crystalline solid below 18°C
Isotropic liquid above 36°C
• Liquid crystal between these two temperatures
• The temperature at which it exhibits L.C. behaviour
can be altered by changing the hydrocarbon tail
• Lyotropic liquid crystals are not pure
substances.
• They are solutions that show liquid crystal
properties at certain concentrations
• For example a solution of soap in water
displays liquid crystal properties at
concentrations above a certain value
• Typically this concentration is “a few percent”
soap!
• At these concentrations micelles form.
• These are when the hyrophobic tails of the
soap molecules all point inwards away from
the water
• And the hydrophilic (polar) heads point
outwards and interact with the water.
C.6.3 Describe the liquid-crystal state in terms of the arrangement
of the molecules and explain thermotropic behaviour.
• In a solid crystal the molecules are fixed in
position and in direction.
• They are held by strong forces.
• If the individual molecules in the crystal are
not symetrical (e.g. they are rod shaped – like
the L.C. molecules we have looked at)
• Then the forces holding the molecules are not
the same in all directions.
• As the crystal is heated, the weak forces are
overcome first.
• This disrupts the order of the crystal in one
direction, but the strong forces still haven’t been
overcome,
• So the molecules still have order in other
directions
• In smectic L.C.s the strong forces keep the
molecules in layers, but the weak forces are
overcome
• Meaning the molecules can move within layers,
so the molecules are only roughly parallel
• If the crystal is heated further, the molecules
gain enough energy to break the layer
structure
• But ON AVERAGE still all point in the same
direction.
• This is a nematic L.C.
• Nematic L.C.s are used in LCDs
• Heating the crystal further, completely breaks
down the order and gives an isotropic liquid.
• (a “normal” liquid)
C.6.4 Outline the principles of the liquid crystal
display device
• Ordinary unpolarized light consists of an
electrical field vibrating in all direction.
• Passing the light through a polarizer only
allows light vibrating on one particular
direction to pass through.
• If a second “crossed” polarizing filter is
introduced, no light can pass through
• Liquid crystals can rotate the direction or
plane of polarized light.
• If a correctly aligned layer of liquid crystal is
placed between the crossed polarizing filters,
then the polarized light is rotated through the
correct angle to pass through the second filter.
• If the L.C. molecule is polar, then its
orientation can be controlled by applying a
small voltage across it.
• Using this technique areas of light and dark
can be produced.
C.6.5 Discuss the properties needed for a substance to
be used in liquid-crystal displays
• In order to be useful in liquid crystal displays,
molecules need to have certain properties.
• They must be chemically stable, and not decompose or
react.
• They must behave as liquid crystals over a suitable
range of temperatures (typically -10°C to +60°C)
• They must be polar (or else we can’t make them
change direction)
• They must be able to change orientation quickly when
a voltage is applied, and change back quickly when the
voltage is removed. This is known as “switching speed”
Review of Syllabus Statements
• C.6.1 Describe the meaning of the term liquid crystals.
• C.6.2 Distinguish between thermotropic and lyotropic liquid
crystals
• C.6.3 Describe the liquid-crystal state in terms of the
arrangement of the molecules and explain thermotropic
behaviour.
• C.6.4 Outline the principles of the liquid crystal display
device
• C.6.5 Discuss the properties needed for a substance to be
used in liquid-crystal displays